spi/bfin_spi: check per-transfer bits_per_word

Currently, if the bits_per_word when doing a transfer is not 8bits, we
always treat it as 16bits when we should actually be returning an error.

Signed-off-by: Bob Liu <lliubbo@gmail.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>